Rats were trained to discriminate (+)-amphetamine sulfate (5.43 mumol/kg, 1 mg/kg) from saline in a food-reinforced, two-lever drug discrimination paradigm. Side chain variations of the amphetamine molecular structure were analyzed for their effects on the discriminative stimulus properties of this prototype central nervous system stimulant. Partial generalization was observed for the alpha-ethyl homologue of (+)-amphetamine, (+)-AEPEA, and for 2-aminoindan (AI), while 5,6-methylenedioxy-2-aminoindan (MDAI) elicited only saline-appropriate responding. By contrast, 2-amino-1,2-dihydronaphthalene (ADN) and 2-aminotetralin (AT) completely substituted for (+)-amphetamine. Relative to the training drug, ADN was 1/4 as potent and AT was 1/8 as potent. The S-(-)-isomer of ADN was found to be responsible for the (+)-amphetamine-like discriminative properties of the racemate. The results suggest that constraining or extending the alpha-alkyl substituent of (+)-amphetamine has a deleterious effect on the ability of the resulting analogue to adopt the active conformation of (+)-amphetamine, thereby diminishing its characteristic discriminative stimulus properties.